Single crystals of metal fluorides such as calcium fluoride and barium fluoride have high transmittance over a wide wavelength region, cause little light scattering and have excellent chemical stability. Therefore, a requirement for them as optical materials, such as lenses and aperture materials of various instruments using laser beam of ultraviolet wavelength or vacuum ultraviolet wavelength, cameras and CVD devices, has been widespread. Particularly, single crystals of alkaline earth metal fluoride, such as calcium fluoride and barium fluoride, are expected as projection lenses used with F2 laser (157 nm) which has been developed as a short wavelength light source of the next generation in the photolithographic technology.
It has been reported that when a single crystal of alkaline earth metal fluoride is used as the projection lens, intrinsic birefringence of the alkaline earth metal fluoride, which causes unclearness of the projected image, can be remarkably reduced by combining a lens taking its optical axis perpendicularly to the {111} plane of the single crystal with a lens taking its optical axis perpendicularly to the {100} plane of the single crystal. As the projection lenses, those having a diameter of not less than 15 cm are generally adopted in order to improve throughput of lithography, and hence, single crystals of large-sized alkaline earth metal fluorides having a diameter of more than 17 cm are required as the lens materials.
Such large-sized single crystals of alkaline earth metal fluorides have been generally produced heretofore by a crucible depression method (Bridgman's method). The crucible depression method is a method wherein a melt of a starting material for forming a single crystal in a crucible is cooled with slowly depressing the crucible containing the starting material to thereby grow a single crystal in the crucible.
In the as-grown single crystal of alkaline earth metal fluoride produced by the crucible depression method, however, there resides a problem that the peripheral surface of the resulting single crystal becomes opaque because the single crystal is formed in such a state that the inner wall of the crucible is in contact with a liquid surface of the starting material melt. As a result, the as-grown crystal obtained has bad visible light transmittance, and therefore, inspection of the internal state, particularly inspection of bubbles or micro-defects included, becomes difficult by visual observation, so that such inspection needs to be carried out after complicated treatment such as cutting or polish of the crystal is performed. Further, especially when a large-sized single crystal of more than 17 cm is grown, the crystal is partially polycrystallized, and hence there is a disadvantage of extremely bad yield.
In order to solve the above problems of the crucible depression method, it is thought that the single crystals of alkaline earth metal fluorides are produced by a single crystal pulling method (Czochralski method). The single crystal pulling method is a method wherein a seed crystal made of the desired single crystal is brought into contact with a melt of a starting material for forming a single crystal placed in a crucible and then slowly pulled from the heating zone of the crucible to cool it and thereby grow a single crystal under the seed crystal. In the single crystal pulling method, the crystal surface is not in contact with the crucible during the growth of a single crystal, so that the surface of the crystal becomes smooth, and besides reduction of impurities due to segregation phenomenon during the crystal growth is possible. Hence, the single crystal pulling method is generally employed for producing semiconductor single crystals such as silicon and germanium.
In the single crystal pulling method, however, the apparatus is complicated, and it is difficult to stably grow crystals, so that in the application of this method to the production of the single crystals of alkaline earth metal fluorides, considerable difficulties are foreseen. With regard to the production of single crystals of alkaline earth metal fluorides by the single crystal pulling method, therefore, an example wherein a small-sized single crystal having a straight barrel part diameter of about 10 cm is produced on a labo-scale is only known (see Shinichiro Tozawa, et al. “Modification of Optical Material CaF2”, report of technical research by Institute for Material Research of Tohoku University, No. 19 (2001), and K. Nassau, Journal of Applied Physics, Vol. 32, 1820-1821 (1961)).
In the actual circumstances, production of a large-diameter single crystal of alkaline earth metal fluoride having a diameter of not less than 17 cm which is made by the single crystal pulling method is rarely known, but in Japanese Patent Laid-Open Publication No. 21197/1999, there is reported a single crystal of alkaline earth metal fluoride of 25 cm diameter obtained by a crucible depression method or a single crystal pulling method using a seed crystal in which the main growth plane and at least one plane of the side planes belong to the same plane orientation ({111} or {100}).
Under such circumstances as mentioned above, the present inventors have attempted to produce a large-diameter single crystal of alkaline earth metal fluoride having a straight barrel part diameter of not less than 17 cm by a single crystal pulling method using such a single crystal pulling apparatus of general structure as described in Japanese Patent Laid-Open Publication No. 21197/1999. However, the peripheral surface of the resulting single crystal in an as-grown state was still considerably opaque. Such opaqueness was solved in the aforesaid production of a small-sized single crystal of alkaline earth metal fluoride by a single crystal pulling method on the labo-scale, and this is a problem having newly occurred with production of a large-sized single crystal of alkaline earth metal fluoride.
The opaqueness lowers visible light transmittance of single crystals as previously described and becomes a serious obstacle to the industrial production of large-diameter single crystals of alkaline earth metal fluorides, so that the solution is desired.
The present invention overcomes the problems and shortcomings of the prior art by providing an as-grown single crystal of alkaline earth metal fluoride having a straight barrel part diameter of not less than 17 cm and having excellent visible light transmittance produced by a single crystal pulling method.